<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  .box{
    width: 200px;
    height: 200px;
    border-radius: 50%;
    background-color: pink;
    animation: ani 4s infinite;
  }

  @keyframes ani {
    0% {
      transform: scale(1);
    }
    25% {
      transform: scale(0.75);
      background-color: yellow;
    }
    50% {
      transform: scale(0.5);
      background-color: red;
    }
    100% {
      transform: scale(0.3);
      background-color: blue;
    }
  }
</style>
<body>
  <div class="box"></div>
  
</body>
</html>